Design High Performance Centrifugal Compressor Vaned Diffusers

The design of the vaned diffuser of a centrifugal compressor is still based on a considerable amount of empiricism in its design methodology. Parameters such as area ratio and blockage, which are more relevant to conical diffusers, are used in conventional design practice.

In Figure 1, the particle paths predicted by a 3D CFD code for a conventionally designed vaned diffuser are presented. The particles are colour coded depending on their point of origin, varying from blue at the hub to red at the tip as shown by the legend. The particle paths indicate a complicated 3D flow pattern in the diffuser, and in particular the presence of a corner separation at the hub. This is due to the accumulation of low momentum fluid at the hub pressure surface corner, which then undergoes a strong positive pressure gradient.

In order to improve the pressure recovery in the diffuser, TURBOdesign1 was used to re-design the vanes using the loading distribution shown in Figure 3. The flow leaving the (conventionally designed) impeller is non-uniform with a high velocity at the hub and a low (almost stagnant) velocity near the shroud wall. In the TURBOdesign1 redesign, a fore-loaded distribution is used at the hub to increase diffusion, and aft-loading at the shroud is employed to avoid any flow separation.

To make the TURBOdesign1 design more challenging, the diffuser was made shorter by placing the trailing edge at r/rtip=1.4 rather than 1.54 for the standard diffuser, and by using 19 vanes rather than 24. In addition the inlet meridional velocity used for the design of the vane was set to vary linearly from hub to shroud.

Fig.1: Predicted particle paths for the conventionally designed impeller.

Fig.2: Predicted particle paths for the impeller designed by TURBOdesign1.

Fig.3: Loading distribution specified in TURBOdesign1 for the design of the diffuser.

Fig.4: Comparison of blade angle distribution of the TURBOdesign1 vane and the conventional vane.

Fig.5: Surface Cp distribution on the conventional vaned diffusers.

Fig.6: Surface Cp distribution on the TURBOdesign1 vaned diffusers.

Fig.7: Comparison of Measured pressure rise characteristic of the stage consisting the same conventional impeller with the TURBOdesign1 and conventional diffuser vanes.

The CFD predictions for the TURBOdesign1 diffuser show that there is no corner separation at the hub (Figure 2). The formation of a corner separation is suppressed as a result of the loading distribution used generating spanwise secondary flows (from shroud to hub), which help to move low momentum fluid away from the hub pressure surface corner.

The TURBOdesign1 vaned diffuser was manufactured from an aluminium disk using a point milling process. The TURBOdesign1 vane and the conventional vane were tested in the same closed loop compressor test facility at the Turbomachinery Laboratory at ETH Zurich. Both vanes were tested with the same conventional impeller. The two pressure rise characteristics are compared in Figure 7. The results show that the TURBOdesign1 diffuser gives a significantly greater pressure rise below design flow conditions, but a similar pressure rise above design flow conditions. A similar choke flow point is obtained in the new design by designing the blades with the same negative incidence (Figure 3).